Fungi produce a vast variety of bio-active secondary metabolites. One group of the metabolites consists of cyclic peptides and depsipeptides, which are often elaborated as families of closely related molecules. The biological activities of these molecules is as varied as their structures, though for convenience, we refer to them as mycotoxins. Because members of a family vary by minor structural modifications, they are ideal subjects for the study of the effect of side chain modification on peptide backbone conformation and, potentially, for the assignment of structure/function relationships. X-ray crystallography will be used in a systematic analysis of the structures and conformations of known peptide and depsipeptide mycotoxins, and as a primary analytical tool for the determination of the structures, absolute stereochemistry, and conformations of novel mycotoxins.